The invention relates to the incorporation of silica powder and aramid polymer fibers into an elastomeric insulation for rocket motor casings.
The combustion of solid rocket propellant creates a hostile environment characterized by extremely high temperature, pressure, and turbulence in the interior of the rocket motor. Temperatures typically exceed 5,000.degree. F. Interior pressure may typically exceed 1,500 psi. Chamber gas velocity typically may reach Mach 0.2 at the aft end of the rocket motor. This environment is considered particularly hostile because the combustion gas typically contains dispersed particles consisting essentially of aluminum oxide liquid droplets. These suspended droplets are believed to produce erosion of the rocket motor case insulation by a sandblasting effect. While the combustion of rocket propellant is usually of short duration, the conditions described above can destroy unprotected rocket motor casings.
Attempts have been made to protect those parts of the rocket structure which may be exposed to the high temperature and erosive flow conditions of the burning propellant. These attempts have principally been directed to methods of protecting the rocket structure by applying some sort of a protective coating or lining to the surfaces which are exposed to the burning propellant and hot turbulent gases. Various plastics and rubbers, both filled and unfilled, have been tried. These include phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins and the like. These plastics are for the most part cured to a rigid structure. Because of the rigid structure of these plastics, cracking and/or blistering is encountered when they are exposed to the rapid temperature and pressure changes encountered during the burning of the propellant. Some rubber-like elastomeric systems have been used. The use of these elastomeric systems is of an ablative nature, that is, the elastomer is sacrificed or consumed in a manner such that the rocket chamber is protected from the high temperature generated during the burning of the fuel.
A more successful method of protecting the structural members of the rocket from these hostile conditions is to provide a lining or coating containing asbestos. This type coating is adapted to withstand flame temperatures and the high velocity flow conditions resulting from the combustion of liquid or solid propellants. Such coatings or linings are capable of enduring for a time sufficient to allow complete combustion of the propellant. Asbestos-reinforced elastomeric insulation systems are the subject of Daly et al, "Elastomeric Composition Containing Chromic Oxide For Use As Rocket Insulation", U.S. Pat. No. 3,421,970 (Jan. 14, 1969), and Hartz et al, "Elastomeric Composition For Use As Rocket Insulation", U.S. Pat. No. 3,347,047 (Oct. 17, 1967).
Environmental and health concerns have led manufacturers to seek a replacement for asbestos-containing rocket motor case insulation which exhibits an acceptably low erosion rate. We attempted to replace asbestos with silica powder, but found that silica reinforced polyisoprene exhibited an unacceptably high erosion rate.